Nuclear medical imaging has gained popularity in recent decades, and medical imaging techniques such as positron emission tomography (PET) scans have been an effective visualization tool in a variety of contexts. For PET, a positron-emitting radionuclide (tracer) comprising a radioactive isotope is typically introduced into the body of a patient. As the radioisotope undergoes beta decay, it emits a positron, which soon interacts with a nearby electron in an annihilation event that produces two gamma photons traveling in opposite directions. The gamma photons may also be referred to as gamma rays or γ-rays. A scintillator or scintillation crystal in a PET scanner typically detects one of the gamma photons originating from the annihilation event, and another scintillator crystal detects the other gamma photon. The scintillator crystals are typically part of detectors that are arranged in a circular or cylindrical configuration around the region where the patient lies. When struck with a gamma photon, each scintillator crystal emits a flash of visible light that is converted to electrons by a photomultiplier tube (PMT) of the PET scanner for subsequent electrical processing.
Based on energy windows that are defined for expected energies of incident gamma photons and based on a check for two coincidental detections at respective scintillators, information about the annihilation event and particularly its location can be stored and processed. In particular, when two gamma photons are captured at respective scintillators, the positron that was annihilated to yield the gamma photons is assumed to have originated somewhere along a line of response (LOR) between the two scintillator crystals. Based on timing information, e.g., respective times of detection at the scintillator crystals corresponding to each detected gamma photon can be used to determine the position of the annihilation event along the line between the scintillator crystals. Because the annihilation event typically occurs very close (e.g., 1 mm) to the site of positron emission from the radioisotope, the location of the tracer that led to the annihilation event can be determined. An image such as a 3D PET image can be computed based on many such gamma photon detections.
Calibration of a PET scanner is an important consideration, as the effectiveness of PET imaging depends in large part on consistency of the images. A PET scanner is a physical system that comprises many pieces of equipment that can go out of calibration over time, e.g., due to drift in gain of PMTs or other causes. Conventionally, calibration of PET scanners is accomplished by a setup procedure that relies on a controlled radioactive source of known, carefully precalibrated radioactivity that decays with a known time constant. Such a radioactive source is typically referred to as a hot phantom. By placing the hot phantom in a controlled, repeatable configuration (e.g., at the center of a gantry of the PET scanner) and measuring gamma detection counts arising from the known radioactivity of the hot phantom, components of the PET scanner can be calibrated. However, such a calibration procedure that relies on an external positron source (phantom) is typically time consuming, as the phantom must be maintained and carefully placed in the center of the gantry. Additionally, calibration that requires a human to handle the phantom repeatedly is associated with health and safety concerns because the phantom is a source of radioactivity.